U.S. patent number 5,563,459 [Application Number 07/613,132] was granted by the patent office on 1996-10-08 for apparatus for controlling opening and closing timings of a switching device in an electric power system.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Hiroshi Arita, Kunio Hirasawa, Yukio Kurosawa, Tadashi Sato.
United States Patent |
5,563,459 |
Kurosawa , et al. |
October 8, 1996 |
Apparatus for controlling opening and closing timings of a
switching device in an electric power system
Abstract
An apparatus for controlling a switching device provided for
breaking or closing a power line connecting a power source to a
load in an electric power system includes a switching timing
control unit for controlling the timing to apply a breaking signal
to said switching device in response to a breaking signal
externally applied thereto so that the contacts of said switching
device are opened during a period of time when the current flowing
through said power line first reaches its peak value from a zero
value and for controlling the timing of applying a closing signal
to said switching device in response to a closing command
preferably externally applied thereto so that the contacts of said
switching device are closed when the voltage of said power source
becomes a predetermined value depending upon whether said load is
capacitive or inductive, and an operating unit for opening and
closing said contacts of said switching device in response to said
breaking and closing signals, respectively.
Inventors: |
Kurosawa; Yukio (Hitachi,
JP), Arita; Hiroshi (Hitachi, JP),
Hirasawa; Kunio (Hitachi, JP), Sato; Tadashi
(Mito, JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
|
Family
ID: |
17815685 |
Appl.
No.: |
07/613,132 |
Filed: |
November 15, 1990 |
Foreign Application Priority Data
|
|
|
|
|
Nov 15, 1989 [JP] |
|
|
1-295051 |
|
Current U.S.
Class: |
307/141.4;
361/195 |
Current CPC
Class: |
H01H
9/56 (20130101); H01H 9/563 (20130101); F02B
2075/027 (20130101) |
Current International
Class: |
H01H
9/54 (20060101); H01H 9/56 (20060101); F02B
75/02 (20060101); H01H 009/56 () |
Field of
Search: |
;361/3,5-7,152,153,185-187,195-202
;307/87,592-598,354,130,131,141.4 ;327/392-400,78 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Fleming; Fritz
Attorney, Agent or Firm: Antonelli, Terry, Stout &
Kraus
Claims
We claim:
1. An apparatus for controlling a switching device provided for
breaking or closing a power line connecting a power source to a
load in an electric power system, said apparatus comprising:
switching timing control means responsive to a breaking command
externally applied thereto for applying a breaking signal to said
switching device at a timing which is controlled so that contacts
of said switching device will be opened during a 1/4 cycle from a
zero value to a peak value of a load current flowing through said
power line; and
operating means for operating said switching device in response to
said breaking signal.
2. An apparatus according to claim 1, wherein said switching timing
control means comprises:
means for detecting the zero value of said load current;
means for storing data representing a time interval from the
detection of the zero value of said load current to generation of
said breaking signal, said time interval being determined so that a
sum of said time interval and an opening time specific to said
switching device falls between two predetermined values related to
a frequency of said electric power system; and
means for generating said breaking signal after lapse of the time
interval represented by the data stored in said storing means from
the detection of the zero value of said load current.
3. An apparatus for controlling a switching device provided for
breaking or closing a power line connecting a power source to a
load in an electric power system, said apparatus comprising:
means for detecting whether said load is capacitive or
inductive;
switching timing control means responsive to a closing command
externally applied thereto for applying a closing signal to said
switching device at a timing which is controlled so that contacts
of said switching device will be closed when a voltage of said
power source attains a value which varies depending on whether said
load is capacitive or inductive; and
operating means for operating said switching device in response to
said closing signal.
4. An apparatus according to claim 3, wherein said switching timing
control means comprises:
means for detecting a zero value of said power source voltage;
means for storing data representing a time interval from the
detection of the zero value of said power source voltage to the
generation of said closing signal, said time interval being
determined so that a sum of said time interval and a closing time
specific to said switching device is equal to a predetermined value
which is related to a frequency of the electric power system and
varies depending on whether said load is capacitive or inductive;
and
means for generating said closing signal after lapse of the time
interval represented by the data stored in said storing means from
the detection of the zero value of said power source voltage.
5. An apparatus according to claim 4, wherein said predetermined
value is determined so that:
the contacts of said switching device will be closed when the power
source voltage attains a zero value when said load is capacitive;
and
the contacts of said switching device will be closed when said
power source voltage attains a peak value when said load is
inductive.
6. An apparatus according to claim 5, wherein said detecting means
detects whether the load is capacitive or inductive based on a load
current flowing through said power line and said power source
voltage when said switching device is closed.
7. An apparatus for controlling a switching device provided for
breaking or closing a power line connecting a power source to a
load in an electric power system, said apparatus comprising:
means for detecting whether said load is capacitive or
inductive;
switching timing control means responsive to a breaking command
externally applied thereto for applying a breaking signal to said
switching device at a timing which is controlled so that contacts
of said switching device will be opened during a 1/4 cycle from a
zero value to a peak value of a load current flowing through said
power line and for applying a closing signal to said switching
device at a timing which is controlled so that the contacts of said
switching device will be closed when a voltage of said power source
attains a value which varies depending on whether said load is
capacitive or inductive; and
operating means for operating said switching device in response to
said breaking and closing signals.
8. An apparatus according to claim 7, wherein said switching timing
control means comprises:
means for detecting the zero value of said load current;
means for detecting a zero value of said power source voltage;
means for storing first data representing a first time interval
from the detection of the zero value of said load current to
generation of said breaking signal, said first time interval being
determined so that a sum of said first time interval and an opening
time specific to said switching device falls between two
predetermined values related to a frequency of said electric power
system, and second data representing a second time interval from
the detection of the zero value of said power source voltage to
generation of said closing signal, said second time interval being
determined so that a sum of said second time interval and a closing
time specific to said switching device is equal to a predetermined
value related to the frequency of said electric power system and
determined so that the contacts of said switching device will be
closed when said power source voltage attains the zero value when
said load is capacitive; and
operating signal generating means responsive to said breaking
command for generating said breaking signal after lapse of the
first time interval represented by said first data stored in said
storing means from the detection of the zero value of said load
current and responsive to said closing command for generating said
closing signal after lapse of the second time interval represented
by said second data stored in said storing means from the detection
of the zero value of said power source voltage when said load is
capacitive.
9. An apparatus according to claim 8, wherein said storing means
also stores third data representing a third time interval from the
detection of the zero value of said power source voltage to the
generation of said closing signal, said third time interval being
determined so that a sum of said third time interval and the
closing time specific to said switching device is equal to a
predetermined value related to the frequency of said electric power
system and determined so that the contacts of said switching device
will be closed when said power source voltage attains a peak value
when said load is inductive; and
wherein said operating signal generating means generates said
closing signal after lapse of the third time interval represented
by said third data from the detection of the zero value of said
power source voltage when said load is inductive.
10. An apparatus according to claim 9, wherein said detecting means
detects whether the load is capacitive or inductive based on the
load current and the power source voltage when said switching
device is closed.
11. An apparatus for controlling an integral three-phase switching
device including three circuit breakers which are connected,
respectively, in three phases of a power line connecting a power
source to a load in a three-phase electric power system and are
operated by a common operating unit, wherein said circuit breakers
open successively at predetermined time intervals when operated by
said common operating unit in response to a breaking command
externally applied thereto, said apparatus comprising:
means for providing a breaking signal for opening said circuit
breakers to said common operating unit in response to said breaking
command; and
switching timing control means for controlling the timing at which
said breaking signal is applied to said common operating unit so
that contacts of one of said circuit breakers which is to be opened
first will be opened during a 1/4 cycle from a zero value to a peak
value of a load current flowing through one phase of the three
phases in which said one circuit breaker is connected.
12. An apparatus for controlling an integral three-phase switching
device including three circuit breakers which are connected,
respectively, in three phases of a power line connecting a power
source to a load in a three-phase electric power system and are
operated by a common operating unit, wherein said circuit breakers
close successively at predetermined time intervals when operated by
said common operating unit in response to a closing command
externally applied thereto, said apparatus comprising:
means for detecting whether said load is capacitive or
inductive;
means for applying a closing signal for closing said circuit
breakers to said common operating unit in response to said closing
command; and
switching timing control means for controlling the timing at which
said closing signal is applied to said common operating unit so
that contacts of one of said circuit breakers which is to be closed
first will be closed when a voltage of one phase of the three
phases in which said one circuit breaker is connected attains a
value which varies depending on whether said load is capacitive or
inductive.
13. An apparatus for controlling an integral three-phase switching
device including three circuit breakers which are connected,
respectively, in three phases of a power line connecting a power
source to a load in a three-phase electric power system and are
operated by a common operating unit, wherein two of said circuit
breakers close substantially at the same time when operated by said
common operating unit in response to a closing command externally
applied thereto, and the remaining circuit breaker closes after a
predetermined time delay from the closing of said two circuit
breakers, said apparatus comprising:
means for detecting whether said load is capacitive or
inductive;
means for applying a closing signal to said common operating unit
for closing said switching device in response to said closing
command; and
switching timing control means for controlling the timing at which
said closing signal is applied to said common operating unit so
that said two circuit breakers will be closed in response to said
closing command when a voltage between two phases of the three
phases in which said two circuit breakers are connected attains a
value which varies depending on whether said load is capacitive or
inductive.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power switching control
apparatus and in particular to a power switching control apparatus
which prevents a phenomenon which gives serious effects to a power
system and devices connected thereto by controlling the switching
timing of a switching device in a power system.
2. Description of the Related Art
Approaches to prevent a transient phenomenon which is severe to
systems and power devices by controlling the switching timing of a
circuit breaker in a power system have heretofore been
proposed.
A Conference Paper No. 13-12 of International Conference on Large
High Voltage Electric Systems held in August-September 1988
entitled "Synchronous Energizing of Shunt Reactors and Shunt
Capacitors"0 discloses that transient inrush currents which are
generated upon energization of shunt reactors or shunt capacitors
can be remarkably reduced by closing a circuit breaker for shunt
reactors at a peak of a voltage of a power source and by closing
the circuit breaker for shunt capacitors at a zero value of the
power source voltage. Since the magnetic flux induced in a core of
a reactor is proportional to the integral of a voltage, the
magnetic flux after 0.5 cycle from the time when the reactor is
energized at the peak value of the power source voltage is just
zero so that the flux is not saturated. Therefore, inrush currents
generated due to saturation of magnetic flux through the core do
not occur in this case. Even if the closing of the circuit breaker
is slightly shifted from the peak point of the voltage, the
saturation of the magnetic flux will be slight and the inrush
currents can be suppressed to a low value within a narrow range. In
energization of the capacitors, the capacitors are energized at a
zero voltage so that no high frequency inrush current will be
generated. Even if the capacitors are energized at a time slightly
different from the point of zero voltage, the voltage applied to
the capacitors is low and the high frequency inrush currents can be
suppressed to a low value within a narrow range. However,
application of this approach to a practical system has technical
problems as follows:
(1) A transient phenomenon occurs on breaking, as well as on
closing. Generation of reignition or restrike will induce an
abnormal voltage. A synchronous energizing technique alone can not
eliminate a possibility of damage in insulation of apparatus.
(2) Circuit breakers are connected with various load devices. This
synchronous energizing approach can be advantageously applied to a
certain device while it may adversely affect on application to
another device resulting in energization under worst conditions. It
is very troublesome to mount different synchronous energizing
devices to different load devices.
SUMMARY OF THE INVENTION
It is a first object of the present invention, in order to overcome
the problem of the prior art, to provide a switching timing control
apparatus for controlling the switching timing of a switching
device for breaking or energizing a load current in a power system,
which eliminates adverse effects due to breaking of the load
current on the power system or devices connected therewith.
It is a second object of the present invention, in order to
overcome the problems of the prior art, to provide a switching
timing control apparatus for controlling the switching timing of a
switching device for breaking or energizing a load current in a
power system, which eliminates adverse effects due to energizing of
the load current on the power system or devices connected
therewith.
According to one aspect of the present invention, an apparatus for
controlling a switching device provided for breaking or closing a
power line connecting a power source to a load in an electric power
system is arranged to comprise switching timing control means for
controlling the timing of applying a breaking signal to said
switching device so that the contacts of said switching device are
closed while the current flowing through said power line changes
from a zero value to its peak value and means for opening said
contacts of said switching device in response to said breaking
signal.
Since the contacts of the switching device are opened in such a
phase that an arc will surely be extinct within a given finite
period of time on breaking of a load current in an apparatus of the
present invention with the above arrangement, an overvoltage across
the contacts of the switching device due to reignition or restrike
can be prevented from being generated irrespective of load
conditions.
According to another aspect of the present invention, an apparatus
for controlling a switching device provided for breaking or closing
a power line connecting a power source to a load in an electric
power system is arranged to comprise switching timing control means
for controlling the timing of applying a closing signal to said
switching device in response to a closing command externally
applied thereto so that the contacts of said switching device are
closed when the voltage of the power source becomes a predetermined
value depending upon the conditions of said load and means for
operating said switching device so that said contacts are closed in
response to said closing signal.
Since the load is energized in a voltage phase preliminarily
determined by a load condition whether the load connected with said
switching device is capacitive or inductive on energization with a
load current in an apparatus of the present invention with the
above arrangement, energizing inrush currents or high frequency
inrush currents can be minimized.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing an embodiment of a control
apparatus of the present invention;
FIG. 2 is a flow chart showing the operation of the apparatus of
FIG. 1;
FIG. 3 is a diagram for explaining the opening timing of contacts
of a switching device in the first embodiment of the present
invention when a load current is broken;
FIG. 4 is a diagram showing the relation between the voltage across
the contacts of the switching device and the dielectric breakdown
voltage on the breaking of the load current; and
FIG. 5 is a table showing switching timings of a switching device
under various conditions.
DESCRIPTION OF THE PREFERRED EMBODIMENT
An embodiment of the present invention will be described with
reference to FIG. 1. In FIG. 1, a circuit breaker 2 functioning as
a switching device is connected with a main circuit 1 which
connects a power source 10 of a power system with a load 20. The
circuit breaker 2 is operated for breaking or closing by a breaking
coil 31 or a closing coil 32, respectively provided in an operating
unit 3. A potential transformer is provided for measuring the input
voltage on the power source side of the circuit breaker. A current
transformer 5 is provided for measuring the load current on the
load side of the circuit breaker. An overcurrent relay 6 is
provided at the output side of the current measuring transformer 5.
A control system of the circuit breaker 2 is provided with a
switching timing control unit 7. The switching timing control unit
7 comprises an A/D converter 71 for analog-to-digital converting
the signals from the potential transformer 4 and the current
transformer 5, a digital input unit 72 for inputting a closing
command for the circuit breaker 2 from a terminal 8 or a breaking
command from a terminal 9, a microprocessor 73, a memory device 74,
a digital output unit 75 for outputting the signals from the
microprocessor 73 and a driver unit 76 for outputting a breaking or
closing command to an operating unit 3 of the circuit breaker 2
according to a digital output from the digital output unit 75.
Operation of the above mentioned switching timing control unit 7
will now be described with reference to FIGS. 2 through 4.
Beginning at a point of switching timing control unit 7 first
determines based on the signals from auxiliary contacts 33 whether
the circuit breaker 2 is on or off (step 200 in FIG. 2). If the
circuit breaker 2 is on or closed, the unit 7 determines from a
signal inputted from the current transformer 4 (step 201 in FIG. 2)
whether or not the current is at a zero value (step 202 in FIG. 2).
If the current is not zero, the program returns to the starting
point A. If the current is zero, the unit 7 determines whether or
not a breaking command is inputted thereto from the terminal 9
(step 203 in FIG. 2). If the breaking command is not inputted, the
program will return to the starting point A. If the breaking
command is inputted, the microprocessor 73 begins to count clock
pulses generated from a clock generator (not shown) incorporated
therein. When the count reaches a preset value corresponding to a
time interval Tc mentioned below (step 204 in FIG. 2), it activates
the digital output unit 75 (step 205 in FIG. 2). This causes the
digital output unit 75 to drive the driver unit 76 (step 206 in
FIG. 2) and to energize the breaking coil 31 of the circuit breaker
2 (step 207 in FIG. 2). As a result of this, the circuit breaker 2
will begin its operation for opening the contacts, which requires
generally a certain opening time Topen specific thereto. The
program returns to the starting point while the circuit breaker
begins to open its contacts. A time chart of this operation is
shown in FIG. 3. The time Tc is determined so that a sum T of the
time Tc counted at step 204 and the opening time Topen specific to
the circuit breaker required for opening its contacts satisfies the
following condition:
wherein f is a frequency of the system to which the circuit breaker
2 is applied and n is an integer. The time Tc as determined based
on the formula (1) is stored in the memory device 74. Accordingly,
the contacts of the circuit breaker 2 surely open at a time between
a zero point of current and its peak point at 1/4 cycle after the
zero point.
FIG. 3 shows a case in which a value of Tc is preset corresponding
to a value T in which n is selected as 2 (n=2) in formula 1. In
this case, the microprocessor 73 commences counting of the clock
pulses at point A. A breaking signal is generated from the digital
output unit 75 at a point B when the count reaches a value
corresponding to a time interval Tc. The circuit breaker commences
separating its contacts at a point C after a lapse of Topen. This
means that an arc time on breaking a load current by the circuit
breaker 2 is surely 0.25 to 0.5 cycle. That is, a gap between the
contacts of the circuit breaker 2 at a time of breaking current
(extinction of the arc) is always a distance corresponding to an
arc time of 0.25 cycle to 0.5 cycle.
In a prior art apparatus, the contacts of the circuit breaker
separate in a zero cycle or a very short arc lasting period of
time. Accordingly, the contacts may not be separated by a large
enough gap length when the arc is extinct. Since the contacts are
separated by a large enough distance at extinction of the arc in
the present invention, the dielectric breakdown voltage between the
contacts can be remarkably increased in comparison with the prior
apparatus.
The relation is shown in FIG. 4. FIG. 4 shows the relation between
the voltage across the contacts and the dielectric breakdown
voltage on breaking. In the drawing, the abscissa denotes time and
the ordinate denotes voltage. Reference numerals 20, 21 and 22
indicate the characteristic curves of the dielectric breakdown
voltage between the contacts when the arc is extinct with the gap
length between the contacts corresponding to arc time of zero, 0.25
and 0.5 cycle, respectively. A curve 23 denotes the voltage applied
across the contacts of the circuit breaker when the load is
capacitive such as a starting capacitor bank, a shunt capacitor, or
a capacitance between cables and between ground and cables. A curve
24 denotes a voltage applied across the contacts immediately after
breaking current when the load is inductive such as a shunt
reactor, an unloaded transformer, a motor and the like. The
dielectric breakdown voltage on breaking a capacitive load with the
arc continuing for substantially zero cycle is required to resist
against a wave (1-cos(.omega.t)) V.sub.0, where V.sub.0 is 1.4
times as high as a phase voltage peak value if the system is an
effectively grounded system since it should meet circuit breaker
standards (for example, JEC-2300 and IEC-Pub 56, etc.). The
dielectric breakdown voltage between the contacts of the circuit
breaker is substantially proportional to the distance of the gap
between the contacts. The dielectric breakdown voltage between the
contacts after 0.5 cycle from the commencement of opening of the
contacts is approximately equal to or more than 3.2 times as high
as the peak value of the phase voltage. Accordingly, the breaking
moment dielectric breakdown voltage after 0.25 cycle of arc time is
not less that 1.6 times and the breaking moment dielectric
breakdown voltage as high after 0.5 cycle of arc time is not less
than 3.2 times as high. Therefore, the dielectric breaking voltage
on breaking a capacitive load always has enough allowance relative
to a voltage practically applied to the circuit breaker. Even if
variations in the opening speed of the contacts and a slight
decrease in dielectric breakdown voltage occur due to arc damage of
the contacts and/or nozzle reignition or restrike will not occur,
resulting in no accidents of dielectric breakdown. Therefore,
reliability can be remarkably enhanced. For example, a
non-effectively grounded system of 3.6 kV to 168 kV is required to
resist against a wave of (1-cos(.omega.t)) V.sub.0 ' where V.sub.0
' is 1.7 times as high as the peak value of the phase voltage for
cycle zero of the arc time of the circuit breaker. The dielectric
breakdown voltages on current breaking (extinction of the arc) in
case of 0.25 and 0.5 cycle of arc time are 1.84 and 3.88 times as
high as the peak value of the phase voltage, respectively, so that
the same advantageous effects are achieved. On breaking an
inductive load, oscillation specific to the inductive load occurs
so that a transient recovery voltage having a relatively high
frequency which is approximately 1 kHz appears between the contacts
of the circuit breaker. For example, the amplitude of the transient
recovery voltage is about 1.5 times as high as the peak value of
phase voltage. The probability that the arc time is 0 to 0.25 cycle
is approximately 50% in case of prior art having no switching
timing control unit. In such a case, the breaking moment dielectric
breakdown voltage between the contacts is not higher than 1.6 times
as the peak value of the phase voltage, resulting in that
reignition almost surely occurs. If the contacts are opened for
breaking the high frequency current generated on reignition,
multiple reignitions in which reignition is repeated many times
would be developed. In the multiple reignitions, energy is stored
in an inductance in a load on each reignition so that the
competition between the increase in dielectric strength between the
contacts due to progress of opening or separation of the contacts
and the increase in amplitude of transient recovery votlage may
cause generation of an excessive surge voltage to breakdown the
insulation of devices. Such a multiple reignition has been
frequently found in gas filled circuit breakers or vacuum circuit
breakers. In the present invention, the dielectric breakdown
voltage between the contacts in the instant of breaking is higher
than 1.6 times so that there is no fear of reignition. Therefore,
no multiple reignition phenomenon is induced and insulation of
devices is not broken. Therefore, a surge absorber which is used to
suppress such an excessive surge voltage may be eliminated. In such
a manner, a very highly reliable power switching apparatus which
effectively suppresses the generation of reignition and restrike
and will not cause damage to the dielectric strength of apparatuses
can be provided in accordance with the present invention.
If an overcurrent is generated due to short-circuiting, the
overcurrent relay 6 is energized to activate the switching timing
control unit 7 for breaking of the circuit breaker 2 in the same
procedure as mentioned above.
Now, contact timing control of the contacts of the circuit breaker
on closing of the circuit breaker 2 will be described.
If it is detected that the circuit breaker 2 is at off (step 200 in
FIG. 2), a voltage derived from the potential transformer 4 (step
208 in FIG. 2) is checked for determination of the zero value (step
209 in FIG. 2). If the inputted voltage is not zero, the program
returns to the starting point A. If it is zero, determination
whether or not a closing command is inputted to the terminal 9
(step 210 in FIG. 2) is performed. If the closing command is not
inputted, the program returns to the starting point A. If the
closing command exists, the microprocessor 73 commences to count
clock pulses (step 211). When the count reaches a preset value
corresponding to the count time TTc which will be described below,
a digital output is generated from the digital output unit 75 (step
212 in FIG. 2) to drive the driver unit 76 (step 213 in FIG. 2) for
energizing the closing operation coil 32 (step 214 in FIG. 2).
After the closing time T.sub.cl specific to the circuit breaker 2
has then passed, the contacts of the circuit breaker 2 are closed.
After a lapse of a sum time TT of the above mentioned count time
TTc and the closing time T.sub.cl specific to the circuit breaker
2, the contacts are closed.
The count time TTc is preset to different values depending upon
whether the load is capacitive or inductive. That is, the value of
TTc is selected so that TT=T.sub.cl +TTc satisfies, when the load
is capacitive,
and, when the load is inductive,
By selecting TTc in such a manner, the circuit breaker can be
synchronously closed. That is, it is possible, in the effectively
grounded power system using phase-independent multi-phase circuit
breaker, to suppress the high frequency inrush currents through
capacitive loads by closing the contacts in each phase of the
circuit breaker at zero point of the phase voltage and suppress the
exciting inrush currents through inductive loads by closing the
contacts in each phase at peak point of the phase voltage. The load
which is connected with the circuit breaker may be capacitive or
inductive. The count time TTc is different for capacitive or
inductive load. It will be suffice to store different count times
TTc depending upon the load of the circuit breaker 2 in the memory
device 74 in the switching timing control unit 7. The load which is
to be connected with the circuit breaker may be changed to a
capacitive or inductive load depending on the power system. In this
case, it will be possible to cope with either of the capacitive or
inductive load by providing a presetting unit which changes the
count time TTc. The presetting unit may be of manual type, or
alternatively automatic type. For example, the automatic type
presetting unit may be operated as follows: As shown in FIG. 2, a
voltage waveform from the potential transformer 4 and a current
waveform from the current transformer are inputted to the
presetting unit (step 215 in FIG. 2), thereby to calculate a power
factor of the circuit. The count time for capacitive load or the
count time for inductive load are calculated (steps 217 and 218,
respectively in FIG. 2). Then these count times are stored (step
219 in FIG. 2) and the count may be preset in the presetting unit
based on the stored count times. Alternatively it is possible to
preset the count by receiving a signal from a suitable controller
for operating the system.
Although the above embodiment has been described with reference to
a circuit breaker of three phases operative independently of each
other, one switching timing control unit may be provided for each
phase, or alternatively one switching timing control unit may be
used commonly for controlling all three phases.
An embodiment in which the present invention is applied to a
circuit breaker of three-phases integratedly operated will now be
described. Firstly, the circuit breaker is formed in such a manner
that the opening time of the contacts for each phase of the circuit
breaker 2 is shifted so as to provide 60.degree. electrical angle
between opening times of the respective contacts of every adjacent
two phases of the circuit breaker. Such a circuit breaker cna be
obtained by changing the length of sliding contact between movable
and stationary contacts of the circuit breaker or by modifying a
link mechanism which forms an operation unit of the circuit
breaker. The switching timing control unit which has been described
with reference to FIG. 1 is applied to a circuit breaker unit which
is connected to a phase which is firstly to be opened. By forming
the circuit breaker in such a manner, it is possible to open the
contacts of the circuit breaker between a current zero point and a
peak point for each phase. It is possible to positively assure 0.25
cycle or arc time for all three phases by mechanically presetting
the circuit breaker so that the contacts are opened in an
electrical angle of 60.degree. from the current zero point in case
of non effectively grounded system. However, it suffices that a
firstly breaking phase does not generate a reignition in a
non-effectively grounded system, since if the second and third
phases are broken in the non-effectively grounded system, a series
breaking having two breaking points is established for breaking the
second and third phases so that a voltage applied to one breaking
point becomes a half. Assuming the order of phases in a system be
A, B and C, the switching timing control unit may be applied to
phase A and the contacts of circuit breakers connected with phases
A and B may be opened at the same time and the contacts of a
circuit breaker connected with phase C may be opened with a delay
of 60.degree.. In case of non-effectively grounded system, a
switching timing control unit may be applied to a circuit breaker
connected with phase A and the contacts of circuit breakers
connected with phases B and C may be opened at the same time with a
time interval of 120.degree. from the opening of the contacts of
the circuit breaker connected with phase A.
Operation of closing is similar to that of breaking. In case of a
circuit breaker used for an effectively grounded system, closing
times of the contacts for the respective phases of the circuit
breaker are shifted to provide 60.degree. in electrical angle
between closings of every adjacent two phases. The switching timing
control unit which has been described with reference to FIG. 1 is
applied to a circuit breaker connected with a first phase to be
firstly closed. By thus forming, it is possible to close contacts
at a zero value of the power source voltage for capacitive loads
and at a peak value for inductive loads. In a non-effectively
grounded system, an unit which delays closing of the contacts of
one phase by 90.degree. in electrical angle from the voltage
between the other two phases is provided so that the switching
timing is controlled based on the phase voltage between given two
phases, for example, phases A and B. If the contacts of the circuit
breakers in phases A and B are closed at a zero value of the phase
voltage between A and B phases in case of a capacitive load, the
circuit breaker of phase C which is delayed by 90.degree. will
close its contacts at a zero value of the intermediate voltage
between the phases A and B, resulting in no generation of high
frequency inrush currents. In case of an inductive load, the
contacts of the circuit breakers for two phases are closed at a
peak value of the phase voltage and the contacts of the circuit
breaker for the remaining phase are closed with a time delay of
90.degree. therefrom. At this time, the voltage of the remaining
phase assumes a peak value with respect to the intermediate of the
phase voltage so that no energizing inrush current is generated.
These closings can sufficiently suppress the energizing inrush
currents and high frequency inrush currents even if the timing
varies more or less due to variations of the closing speed of the
contacts and a leading arc.
Operation of the above mentioned synchronous switching is
summarized in a table shown in FIG. 5.
The above mentioned switching timing control unit 7 may be provided
in a main body of the circuit breaker or alternatively its
functions may be incorporated in the overcurrent relay 6. The
switching timing control unit 7 may be made of a one chip LSI. It
is not always necessary to use a microprocessor and other logic
circuits may be used in lieu of the microprocessor.
Since the present invention sets the opening timing of the contacts
of the circuit breaker during an interval between a zero point and
a peak point of a breaking current where an absolute value of the
current is increased as mentioned above, the arc period of time is
always not less than 0.25 cycles so that a sufficient opening and
separation length between the contacts may be assured for
increasing the dielectric breakdown voltage at the current-breaking
instant. As a result of this, reignition and restrike can be
satisfactorily suppressed. This prevents an excessively high
overcurrent from being generated. Therefore, a power switching
apparatus having a high reliability which will not break down
insulation of devices can be provided.
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